Some food for thought.
Are you sure you have the right b.date? It's not impossible but it seems off that there would be two Mary Mannix in Manitoba with October 14th births. As well, the following is the only Mary Mannix I can locate in Canada in the 1901 census.
Is it possibly this family in 1901? I realize that Mary's b.d. is out by 1 year but it's the right month and day.
1 21 4 Mannix Charles M Head M Feb 4 1869 32, b. QU r, Irish, rancher
1 22 4 Mannix Louise F Wife M Mar 5 1870 31, b. England r, emig 1888
1 23 4 Mannix Mary L F Daughter S Oct 14 1898 2, b. MB r, Irish
1 24 4 Mannix William N M Son S Apr 29 1899 1, b. MB r
1 25 4 Mannix Annie A F Daughter S Jan 12 1901, b. MB r
1 26 4 Mannix Wm. M Father W May 10 1839 61, b. England (I think) r, emig 1882
1 27 4 Mannix Arthur M Son S Mar 11 1877 24, b. QU r
1 28 4 Bullock Ernest M hired boy S Feb 8 1888 13
1901 Census of Canada
Subdistrict: Woodlands, SELKIRK, MANITOBA
District Number: 11
Subdistrict Number: p-2
Archives Microfilm: T-6435
There's also this birth which is one year too early but appears to be the Mary L above, because the mother is listed as Louisa Nash.
LAST NAME: MANNIX
FIRST NAME: MARY LOUISE
DATE OF BIRTH: 10/14/1897
PLACE OF BIRTH: R.M. OF ROCKWOOD
REGISTRATION NUMBER: 1897,003667
In 1906, the above children are listed as:
Mary Louise 8
Ellen 5 mons
but the Charles & Louise now give their birth place as Manitoba. (maybe just a census taker error because Charles can be found in the 1881 census living in Bury, Compton, Quebec with an Irish father and an English mother).